A grease composition is enclosed in a rolling bearing incorporated in various industrial machines, vehicles, and the like to impart lubricating properties to the rolling bearing. The grease composition is obtained by compounding a thickener with a base oil. As the base oil, mineral oil and synthetic lubricating oil such as ester oil, silicone oil, ether oil, fluorine oil, and the like are generally used. As the thickener, metal soap such as lithium soap, urea compounds, and fluororesin are generally used. In recent years, a rolling bearing tends to be used at a high temperature and a high rotational speed. Thus the grease composition is demanded to have durability at a high temperature. When the metal soap is used as the thickener, oxidation of the base oil is accelerated at the high temperature and thus lubricating effect deteriorates. Regarding the base oil, the grease composition containing the mineral oil as the base oil thereof is oxidized more readily than the grease composition containing the synthetic lubricating oil as the base oil thereof and thus tends to have a short lubricating life at the high temperature. Thus in recent years, the grease composition containing the synthetic lubricating oil as the base oil thereof and the urea compound or the fluororesin as the thickener thereof is mainly used for the rolling bearing which is used especially at a high temperature and at a high rotational speed.
As various industrial mechanical members become compact and have high performance, use conditions of the rolling bearing tend to be stricter. Consequently the grease composition is demanded to have a higher lubricating properties and a longer lubricating life.
In compliance with the demand for the longer lubricating life at a high temperature, the addition of an antioxidant and a rust preventive agent to the grease composition containing a synthetic lubricating oil having a high viscosity as its base oil and an urea compound as its thickener is examined. But when the grease composition contains the additives, the viscosity of the grease composition rises under cold condition, thereby the grease composition is liable to cause an abnormal noise.
When an equipment to be driven by an automotive engine such as a pulley is operated under cold condition, there is a case in which an abnormal sound (sound similar to that generated by playing flute), so-called a low-temperature abnormal noise, is generated. Although the reason why the low-temperature abnormal noise is generated has not been clarified, it is supposed that the abnormal noise is generated owing to a self-excited vibration of a rolling element caused by nonuniformity of an oil film of grease. Specifically, under cold condition, the oil film on the raceway surface is liable to be nonuniform owing to a rise of the viscosity of the base oil of the grease. The nonuniformity of the oil film causes the coefficient of friction between the rolling element and the raceway surface to make a slight periodical change. Thereby the rolling element generates the self-excited vibration. Owing to the self-excited vibration, the pulley system resonates with the rolling element. As a result, the outer ring vibrates axially (translational motion) to generate the low-temperature abnormal noise.
As the grease excellent in the durability at the high temperature and preventing the generation of the low-temperature abnormal noise, grease containing a base oil composed of a mixed oil of synthetic hydrocarbon oil and ester synthetic oil having eight or more ester groups arranged in a comb-like fashion on one side of eight or more carbon atoms constituting chain molecules of oil, an urea compound mixed with the base oil as a thickener, and dithiophosphate added to the base oil as an extreme-pressure agent is known (see patent document 1).
A bearing for use in an automotive pulley which includes a grease composition containing a base oil composed of a mixed oil of poly-α-olefin (hereinafter abbreviated as PAO) oil and an ester oil, an alicyclic diurea compound mixed with the base oil as a thickener, and zinc dithiocarbamate as an additive added to the base oil is known. In the bearing, a contact angle is formed by bringing each of a plurality of balls interposed between an inner ring and an outer ring both constituting the bearing into contact with the inner ring or with at least the outer ring at two points (see patent document 2).    Patent document 1: Japanese Patent Application Laid-Open No. 9-208982    Patent document 2: Japanese Patent Application Laid-Open No. 11-270566
These attempts are aimed at the stability of the oil film under cold condition to prevent the generation of the low-temperature abnormal noise, and at a long life of the grease composition at the high temperature. But only the mixture of the synthetic hydrocarbon oil and the ester oil is incapable of obtaining an effect of sufficiently preventing the generation of the low-temperature abnormal noise.